A research of a photo imaging technology for irradiating a living body with light from a light source such as a laser and imaging information of an internal body obtained on the basis of the incident light is advanced in a medical field. As one of the photo imaging technologies, Photo Acoustic Imaging (PAI) is proposed. In the photo acoustic imaging, a living body is irradiated with pulsed light generated from a light source. An acoustic wave (typically, an ultrasonic wave) generated from a body tissue that has absorbed energy of the pulsed light which is propagated and diffused in the living body is detected, and living body information is imaged on the basis of the detected signal.
That is, in the photo acoustic imaging, a difference in a rate of absorption for light energy between a subject part such as a tumor and other tissues is utilized, and an elastic wave (which is also referred to as photoacoustic wave) generated when the subject part absorbs the emitted light energy and instantaneously expands is detected by a photoacoustic wave detector (which is also referred to as transducer or probe). While this detection signal is subjected to an analysis process, it is possible to obtain an image of the optical characteristic value distribution.
In addition, by measuring these pieces of information with lights at various wavelengths, it is also possible to utilize the information in quantitative measurements for particular substances in the subject (for example, a concentration of hemoglobin included in blood, a degree of oxygen saturation of blood, and the like).
As described above, the subject is irradiated with the pulsed light in the photo acoustic imaging. The light is diffused in the subject because of a strong optical scattering characteristic, and photoacoustic waves are generated from a wide sphere at the same time. As a result, a resolution of the photoacoustic image is decreased.
In view of the above, to solve this problem, a technique of evaluating signals at generation locations of the photoacoustic waves detected by the respective photoacoustic wave detectors is adopted. According to this technique, with respect to the signals spreading in a concentric fashion, if the respective photoacoustic wave detectors can receive the same signal intensity, it is determined that the signal is a signal from an optical absorber. On the other hand, if the respective photoacoustic wave detectors do not receive the same signal intensity, it is determined that the signal is a noise signal. As an example of this technique, a factor called Coherence Factor (CF) is utilized according to NPL 1. The CF is a value calculated in the following expression for each area after dataSi(1≤i≤N)  [Math.1]observed by each of N pieces of photoacoustic wave detectors is allocated to the respective areas through a back projection method such as Circular back projection.
                    [                  Math          .                                          ⁢          2                ]                                                            CF        =                                            (                                                ∑                                      i                    =                    0                                                        N                    -                    1                                                  ⁢                                                                  ⁢                                  S                  i                                            )                        2                                N            ⁢                                          ∑                                  i                  =                  0                                                  N                  -                  1                                            ⁢                                                          ⁢                                                (                                      S                    i                                    )                                2                                                                        (        1        )            
According to the above-mentioned expression, in an area where the photoacoustic wave is generated, the CF is close to 1. In an area where the photoacoustic wave is not generated or an area where random noise is observed, the same signal intensity is not detected by the respective photoacoustic wave detectors, and the CF is close to 0. For that reason, since the CF becomes an index indicating a variation rate of the signals observed from the respective transducers for each area, it is possible to increase a reliability of the signals by weighting the respective signals with the CF as a coefficient. When the thus weighted signals are used, an improvement in the resolution can be realized for reconstructing the image, and it is also possible to decrease an influence of artifacts.